This application claims the priority of Japanese Patent Applications No. H9-105490 filed on Apr. 23, 1997, H9-106975 filed on Apr. 24, 1997, H9-107141 filed on Apr. 24, 1997, H9-257542 filed on Sep. 5, 1997 and H9-362693 filed on Dec. 12, 1997, and Japanese Patent Application No. H10-104158 filed on Mar. 30, 1998 with the title of the invention of xe2x80x9cResistor-Incorporated Spark Plug, Resistor Composition for Spark Plug and Manufacturing Method of Resistor-Incorporated Spark Plugxe2x80x9d, which are incorporated herein by reference.
The present invention relates to spark plugs to be used for internal combustion engines and, more particularly, to a spark plug into which a resistor for prevention of occurrence of radio frequency noise is incorporated and the manufacturing method thereof.
As this type of spark plug, there has conventionally been known one having a structure that a terminal is fixed in one end portion of a through hole formed along the axial direction of an insulator while a center electrode is similarly fixed in the other end portion of the through hole, where a resistor is placed between the terminal and the center electrode within the through hole. This resistor is implemented by one which is formed through steps of mixing amorphous carbon (e.g., carbon black) into glass powder and/or dielectric ceramic powder and thereafter sintering the mixture by hot press or the like as shown in Japanese Patent Laid-Open Publication S61-104580, S61-253786, or H2-126584.
In this connection, recently internal combustion engines such as automobile engines are on the trend toward higher output, while power supply ability has been on the increase for improvement of ignitionability. Also, with the downsizing of internal combustion engines, resistor-incorporated spark plugs have also been required to be smaller in size and higher in performance. Under these circumstances, there is an issue that when some high load is applied on such a resistor-incorporated spark plug, particularly on a small-size spark plug with a small-diameter resistor, the carbon that imparts electrical conductivity to the resistor would burn, causing the resistance value to increase, so that a stable load life characteristic could not be obtained.
An object of the present invention is to provide a resistor-incorporated spark plug, as well as a manufacturing method therefor, which is enabled to offer a stable load life characteristic even when a high load is applied thereon, and also to provide a resistor composition to be used for the resistor-incorporated spark plug.
Resistor-incorporated spark plugs according to the present invention have the following common structure in their essential part. That is, with respect to a through hole formed along an axis of an insulator, a terminal is fixed to one end side of the through hole while a center electrode is fixed to the other end side of the through hole, and a resistor made of a resistor composition principally comprising a conductive material, glass particles and ceramic particles other than glass is placed between the terminal and the center electrode within the through hole. Further, in a first constitution of the resistor-incorporated spark plug of the present invention, the resistor composition contains, as the ceramic particles, semiconductive ceramic particles, and (xcex12xe2x88x92xcex11)/xcex11xe2x89xa7xe2x88x920.30 where a value of electric resistance measured by making conduction between the terminal and the center electrode via the resistor is xcex11 at 20xc2x0 C. and xcex12 at 150xc2x0 C.
In an attempt to improve the load life characteristic of the spark plug, proposals for stabilizing the load life of the resistor by blending TiO2 particles, which is a semiconductor oxide, in the resistor have been disclosed, for example, in Japanese Patent Laid-Open Publications S58-102480, S58-102481, S58-189917, S59-17201, S59-17202, S60-150601, S60-150602 and Japanese Patent Publication H5-52641. However, with higher output of an internal combustion engine, the spark plug used as it is attached to the internal combustion engine may increase in temperature so that the resistor built in the spark plug also increases in temperature, for example, to as high as about 100-300xc2x0 C. When such a state occurs, the electric resistance of the semiconductive TiO2, and moreover the specific electrical resistivity of the resistor, decreases so that the radio frequency noise prevention performance (radio frequency noise performance) is impaired, as a disadvantage.
Therefore, according to the first constitution of the invention, in the spark plug in which semiconductive ceramic particles are blended in the resistor, a condition of (xcex12xe2x88x92xcex11)/xcex11xe2x89xa7xe2x88x920.30 is satisfied, where the value of electric resistance measured by making conduction between the terminal and the center electrode via the resistor is xcex11 at 20xc2x0 C. and xcex12 at 150xc2x0 C. By this setting, enough radio frequency noise prevention performance can be obtained even at high temperatures. In addition, if (xcex12xe2x88x92xcex11)/xcex11 less than xe2x88x920.30, then the noise prevention performance at high temperatures may become insufficient. It is more preferable to satisfy that (xcex12xe2x88x92xcex11)/xcex11xe2x89xa7xe2x88x920.27.
Next, it is possible that the resistor composition contains, as the semiconductive ceramic particles, 0.5-20 weight % of TiO2 particles whose mean particle size of a particle image obtained from observation of its cross-sectional structure falls within a range of 0.5-20 xcexcm, the TiO2 particles at least partly having a rutile type crystalline structure. It is noted that although every metal oxide herein is represented by a composition formula having a stoichiometric composition, there are some cases actually where the metal oxide becomes a nonstoichiometric composition due to oxygen deficiency.
With this constitution, a successful load life characteristic can be ensured even under a high load condition by 0.5-20 weight % of TiO2 particles being contained in the resistor composition. Further, by adjusting the mean particle size of the TiO2 particles to be blended in the resistor composition within a range of 0.5-20 xcexcm so that the TiO2 particles at least partly have a rutile type crystalline structure, high-temperature deterioration of the radio frequency noise prevention performance by the resistor can be effectively suppressed.
For example, when the resistor contains TiO2 particles and a non-metallic conductive material such as carbon particles, its conduction path is formed by contact between non-metallic conductive material particles themselves, between non-metallic conductive material and TiO2 particles or between TiO2 particles themselves. Besides, it is considered that the electric resistance value of the resistor can be expressed by a sum of intrinsic resistance (bulk resistance) of these particles and contact resistance between the particles.
In this connection, as a result of the present inventors"" energetic researches, it was found out that the temperature dependency of a resistor as described above is ruled mainly by temperature variation in the intrinsic resistances of the individual particles. Meanwhile, TiO2 has been known to have three types of crystalline structures under atmospheric pressure phase, the rutile type of the tetragonal system, anatase type of the tetragonal system and brookite type of the rhombic system. Among these, the two types of the rutile type and the anatase type are of industrial importance. The above constitution of the present invention has been completed by focusing on the fact that, in these two types of TiO2, the rutile type results in smaller temperature variations of specific resistance than the anatase type.
If the content of TiO2 particles in the resistor composition is less than 0.5 weight %, then the resistor becomes insufficient in load life characteristic. If it exceeds 20 weight %, its noise prevention performance is more likely to deteriorate due to high temperature. It is preferable to adjust the content of TiO2 particles in the resistor composition desirably to 2-20 weight %, and more desirably to 3-15 weight %.
It can be generally said for TiO2 that the anatase type crystalline structure tends to become more stable with decreasing particle size. Besides, if the mean particle size of TiO2 particles is less than 0.5 xcexcm, then the noise prevention performance by the resistor becomes more likely to deteriorate due to high temperature, which leads to a deterioration in temperature characteristic of the noise prevention performance. This could be attributed to the fact that the TiO2 particles contained are formed into finer particles so that the relative content of the anatase type phase increases and, in turn, the relative content of the rutile type phase lacks, thus resulting in insufficient temperature characteristic of the noise prevention performance. As another problem, if the mean particle size of TiO2 particles is less than 0.5 xcexcm, then the bulk density of TiO2 raw material powder increases so that the density of the resistor obtained by firing becomes insufficient, which leads to impairment of the noise prevention performance or load life characteristic. On the other hand, if the mean particle size of TiO2 particles exceeds 20 xcexcm, then the raw material powder particles of the resistor including not only TiO2 powder but also later-described glass powder and ceramic powder other than TiO2 become less easy to rearrange in the firing process, which leads to insufficient density of the resistor as well. The mean particle size of the TiO2 particles in the resistor composition is preferably adjusted within a range of, more desirably, 2-8 xcexcm.
Next, desirably, 20 weight % or more of the TiO2 particles in the resistor composition have the rutile type crystalline structure (rutile type phase). In this case, the rest of the TiO2 particles may be those having the anatase type crystalline structure (anatase type phase). If the content ratio of the rutile type phase occupying in the total amount of TiO2 is less than 20 weight %, then temperature characteristic of the noise prevention performance may become insufficient. The content ratio of the rutile type phase is more desirably not less than 30 weight %. It is also preferable to adjust the content ratio of the rutile type phase to not more than 80 weight %. The rutile type phase being generally coarser than the anatase type phase, if the content ratio of the rutile type phase exceeds 80 weight %, then the conduction path forming part formed in the resistor principally of TiO2, later-described metallic phase or non-metallic conductive material becomes non-uniform so that a stable load life characteristic cannot be obtained in some cases. The content ratio of the rutile type phase is, more desirably, not more than 70 weight %.
Also, with regard to its particle size distribution, it is preferable in terms of ensuring stable load life characteristic and noise prevention performance that a content ratio of the TiO2 particles belonging to a particle size range of 0.05-0.5 xcexcm is 20-80 weight %, and a content ratio of the TiO2 particles belonging to a particle size range of 2-8 xcexcm is 80-20 weight %. That is, the TiO2 particles belonging to the particle size range of 2-8 xcexcm are, in most part, those mainly having the rutile type phase, where its content ratio of 20 weight % or more makes it possible to attain a good temperature characteristic of the noise prevention performance. Further, when the content ratio of TiO2 particles belonging to the particle size range of 2-8 xcexcm is not more than 80 weight %, and when the content ratio of TiO2 particles belonging the particle size range of 0.05-0.5 xcexcm is adjusted to the aforementioned range, the conduction path forming part in the resistor can be made uniform in thickness so that a stable load life characteristic can be obtained. In addition, the content ratio of TiO2 particles belonging to the particle size range of 0.05-0.5 xcexcm is more desirably 30-70 weight %, and the content ratio of TiO2 particles belonging to the particle size range of 2-8 xcexcm is more desirably 70-30 weight %.
Next, the resistor composition may contain, as the semiconductive ceramic particles, 0.5-20 weight % of at least either one of a semiconductive titanate base complex oxide and a semiconductive zirconate base complex oxide (hereinafter, referred to as specific complex oxide when generically designated).
This constitution has been completed by focusing on the fact that both titanate base complex oxide and zirconium base complex oxide are small in intrinsic resistance variation with temperature, as compared with TiO2 that has conventionally been used as the semiconductive ceramic particles. Then, a successful load life characteristic can be ensured even under a high load condition by 0.5-20 weight % of TiO2 particles being contained in the resistor composition and besides high-temperature deterioration of the noise prevention performance by the resistor can be effectively suppressed.
If the content of the specific complex oxide in the resistor composition is less than 0.5 weight %, then the resistor becomes insufficient in load life characteristic. If it exceeds 20 weight %, its noise prevention performance is more likely to deteriorate due to high temperature. It is preferable to adjust the content of the specific complex oxide in the resistor composition desirably to 2-20 weight %, and more desirably to 3-15 weight %.
As the aforementioned specific complex oxide, titanates of alkaline-earth metal elements or zirconates of alkaline-earth metal elements are particularly preferably usable for the present invention by virtue of their having successful semiconductor characteristics and their small variations in specific resistance with temperature.
Such titanates or zirconates of alkaline-earth metal elements can be exemplified by magnesium titanate (composition formula: MgTiO3, which however may be a nonstoichiometric composition due to oxygen deficiency, also for the following), magnesium zirconate (composition formula: MgZrO3), calcium titanate (composition formula: CaTiO3), calcium zirconate (composition formula: CaZrO3), strontium titanate (composition formula: SrTiO3), strontium zirconate (composition formula: SrZrO3), barium titanate (composition formula: BaTiO3) and barium zirconate (composition formula: BaZrO3). For the present invention, one kind or more selected from among these may used singly or in combination.
It is preferable that the mean particle size of particles of the specific complex oxide in the resistor composition is adjusted within a range of 0.5 xcexcm-20 xcexcm. If the mean particle size is less than 0.5 xcexcm, then the bulk density of the specific complex oxide raw material powder increases so that the density of the resistor obtained by firing lacks, which may impair the noise prevention performance or the load life characteristic. On the other hand, if the mean particle size of the specific complex oxide exceeds 20 xcexcm, then the raw material powder particles of the resistor including not only the specific complex oxide but also later-described glass powder and ceramic powder other than the specific complex oxide become less easy to rearrange in the firing process, which may resulting in insufficient density of the resistor as well. The mean particle size of the specific complex oxide in the resistor composition is preferably adjusted within a range of, more desirably, 2-8 xcexcm.
In the above constitution, it is preferable that content of a remainder of the ceramic particles from which the TiO2 particles or the specific complex oxide particles (hereinafter, referred to as auxiliary ceramic particles) is 2-32 weight %. If the content of the auxiliary ceramic particles falls outside the above range, the load life characteristic of the spark plug may be impaired. The content of the auxiliary ceramic particles is desirably adjusted within a range of 3-20 weight %. The auxiliary ceramic particles may be those composed principally of, for example, one kind or more selected from a group of ZrO2, ZrSiO4, Al2O3, MgO, Alxe2x80x94Mg spinel and mullite.
The resistor composition may be one which contains 2-90 weight % of glass, 2.5-52 weight % of ceramic particles (including TiO2 particles or specific complex oxide) and 0.1-5 weight % of carbon component. Such a resistor composition can be obtained, for example, by preparing a raw material powder through the steps of mixing 2-90 weight % of glass powder, 2.5-52 weight % of ceramic particles, a non-metallic conductive material (e.g., carbon black) and 0.1-5 weight % of organic binder (e.g., PVA) plus, as required, an appropriate amount of metal powder (which results in a metallic phase), and heating and molding this raw material powder.
More specifically, the resistor composition can be produced by blending and hot pressing 3-20 weight % of glass particles having a mean particle size of less than 150 xcexcm (hereinafter, referred to as fine particles), 60-90 weight % of glass particles belonging to a particle size range of 150-800 xcexcm (hereinafter, referred to as coarse-particle glass), 0.5-20 weight % of TiO2 particles or specific complex oxide particles, 2-32 weight % of auxiliary ceramic particles, 0.05-0.5 weight % of a metal powder composed principally of one kind or more selected from a group of Al, Mg, Ti, Zr and Zn (which forms a metallic phase) and 0.5-5.0 weight % of non-metallic conductive material powder.
FIG. 4 schematically shows the structure of the above resistor composition obtained in this way. That is, at least part of the fine-particle glass is melted and then solidified to form a binding glass phase, into which the metallic phase and the non-metallic conductive material particles (hereinafter, referred to generically as conductive material powder), thus forming a conduction path forming part. The conduction path forming part forms a so-called block structure, surrounding block glass particles which originate from the coarse-particle glass. In this case, at least part of the binding glass phase forms a continuous portion which ranges from the terminal-side end portion on the center-electrode-side end portion, where the continuous portion forms the conduction path of the resistor based on electrical contact between particles themselves of the conductive material powder. This continuous portion, or conduction path, is bypassed all over by the intervention of the block particles so that its effective length is elongated, by which a successful radio frequency noise prevention effect can be achieved.
The fine-particle glass at least partly melts during the hot press process, playing a role of filling gaps formed between the particles themselves of the fine-particle glass powder. However, if its particle size exceeds 150 xcexcm, the fine-particle glass melts insufficiently so that voids tend to occur to the conduction path, which leads to impairment of the load life characteristic of the spark plug. In addition, it is preferable to set the particle size of the fine-particle glass powder within a range of, desirably, not more than 100 xcexcm. In the case of the coarse-particle glass, on the other hand, if its particle size is less than 150 xcexcm, the particles becomes more likely to soften or melt during the heating and molding process, so that the aforementioned block structure is impaired, making it impossible to achieve a successful noise prevention effect. Also, if the particle size exceeds 800 xcexcm, voids are more likely to remain among the glass particles, which leads to impairment of the load life characteristic of the spark plug.
Further, if the weight of the fine-particle glass is less than 3 weight %, or if the weight of the coarse-particle glass exceeds 90 weight %, then the glass comes to hardly melt during the hot press process, so that a great deal of voids are formed between glass particles, causing an impairment of the load life characteristic. On the other hand, if the weight of the fine-particle glass exceeds 30 weight %, or if the weight of the coarse-particle glass is less than 60 weight %, then the content ratio of the block particles decreases so that the formation of the block structure becomes insufficient, making it impossible to achieve a successful radio frequency noise prevention effect. In addition, it is preferable to set the weight of the fine-particle glass within a range of, desirably, 3-12 weight %. Also, it is preferable to set the weight of the coarse-particle glass within a range of, desirably, 70-85 weight %.
If the blending amount of the metallic phase or the non-metallic conductive material deviates from the upper limit value of the above range, there are some cases where the radio frequency noise prevention effect becomes insufficient. Conversely, if it deviates from the lower limit value, there are some cases where the load life characteristic is impaired. The blending amount of the metallic phase is preferably adjusted within a range of, desirably, 0.1-0.3 weight %, and the blending amount of the non-metallic conductive material is preferably adjusted within a range of, desirably, 0.5-3.0 weight %.
Also, from the viewpoint of structure, the resistor composition is preferably constituted as follows. That is, the resistor composition comprises:50-90 volume % of block glass particles comprising particles belonging to a particle size range of 150-180 xcexcm; and 10-50 volume % of conduction path forming part which contains the conductive material, the ceramic particles and a binding glass phase for binding the conductive material and the ceramic particles with each other in their dispersed state, and which has such a form as to fill gaps among the block glass particles, and further which forms a conduction path within the resistor.
If the content ratio of the block glass particles is less than 50 volume %, or if the content:ratio of the conduction path forming part itself in the resistor composition exceeds 50 volume %, then the formation of the block structure becomes insufficient so that a successful radio frequency noise prevention effect cannot be achieved. Conversely, if the content ratio of the block glass particles exceeds 90 volume %, or if the content ratio of the conduction path forming part itself in the resistor composition is less than 10 volume %, then a great deal of voids are formed between the glass particles, causing an impairment of the load life characteristic of the spark plug. It is preferable that the content ratio of the block glass particles is adjusted within a range of, more desirably, 20-40 volume %.
It is noted that the particle size of block glass particles is defined as a maximum value d of the distance between two parallel lines A, B, as shown in FIG. 8, where the parallel lines A, B are drawn, with respect to an outline of a particle on a resistor cross section, so as to be tangential to the outline and not to cross the inside of the particle, in various patterns by varying the positional relation with the particle (the case is similar also to the particle size of TiO2 particles and the like as described before). Then, the volume content ratio of the block glass particles can be calculated by dividing the total area of the block glass particles observed on the resistor cross section by the field-of-view area.
The conductive material contained in the conduction path forming part may contain, for example, a metallic phase principally comprising one kind or more selected from among Al, Mg, Ti, Zr and Zn, and a non-metallic conductive material.
Also, the conduction path forming part may contain 7.5-50 weight % of the binding glass phase, 0.1-3.0 weight % of the metallic phase, 1.2-12.5 weight % of the non-metallic conductive material, 5-80 weight % of ceramic particles in which the TiO2 particles or the specific complex oxide particles occupy 5-50 weight %, on a basis of weight content ratio occupying in the conduction path forming part.
If the content ratio of the binding glass phase in the conduction path forming part is less than 7.5 weight %, then the glass comes to hardly melt during the hot press process, so that a great deal of voids are formed between glass particles, causing an impairment of the load life characteristic of the spark plug. On the other hand, if it exceeds 50 weight %, then the relative ratio of the metallic phase or the non-metallic conductive material decreases, which leads to an impairment of the load life characteristic. Also, if the content ratio of the metallic phase or the non-metallic conductive material particles in the conduction path forming part deviates from the upper limit value of the above range, the radio frequency noise prevention effect becomes insufficient in some cases. Conversely, if it deviates from the lower limit value, the load life characteristic is impaired in some cases.
Further, if the content ratio of the TiO2 particles or the specific complex oxide particles in the conduction path forming part is less than 5 weight %, then the resistor becomes insufficient in load life characteristic. If it exceeds 50 weight %, its noise prevention performance is more likely to deteriorate due to high temperature. In this case, the volume ratio of the TiO2 particles or the specific complex oxide particles occupying in the conduction path forming part is preferably adjusted within a range of 5-50 volume %, desirably, of 20-40 volume % because of the same reasons. In addition, this volume ratio VR can be calculated, for example, by the following equation:
VR={S0xc3x97V1/(V1+V2)}xc3x97100(volume %)xe2x80x83xe2x80x83(1)
where S0 is the area ratio of ceramic particles observed in a cross-sectional structure of the resistor composition, V1 is the volume of the TiO2 particles or the specific complex oxide particles contained in the resistor composition identified by X-ray diffraction and V2 is the volume of the auxiliary ceramic particles determined likewise.
The non-metallic conductive material may be provided as one principally comprising one kind or more selected from the particles of amorphous carbon (carbon black), graphite, SiC, TiC, WC and ZrC. In this case, the resistor composition contains a carbon component based on the non-metallic conductive material, where the carbon component presents mainly in the conduction path forming part. For example when carbon black is used, at least part of the carbon component is contained in the conduction path forming part in the form of carbon black particles.
Preferably, the content of the carbon in the resistor composition is adjusted within a range of 0.5-5 weight %. If the carbon content is less than 0.5 weight %, then the load life characteristic of the spark plug is impaired in some cases. Also, if the carbon content exceeds 5 weight %, then the radio frequency noise prevention effect becomes insufficient in some cases. The carbon content is preferably adjusted within a range of, more desirably, 0.5-3 weight %. In addition, there are some cases where carbon components originating from organic binders for use of powder molding are contained in the non-metallic conductive material.
For the present invention, the material of the glass particles may be one containing one kind or more selected from a group of, for example, B2O3xe2x80x94SiO2 based, BaOxe2x80x94B2O3 based, SiO2xe2x80x94B2O3xe2x80x94CaOxe2x80x94BaO based, and SiO2xe2x80x94ZnOxe2x80x94B2O3 based, SiO2xe2x80x94B2O3xe2x80x94Li2O based, and SiO2xe2x80x94B2O3xe2x80x94Li2Oxe2x80x94BaO based glass powders. In this case, if a material having a softening temperature of not more than 800xc2x0 C. is used, the glass is enhanced in its fluidity at the melting so that the binding glass phase spreads enough to the gaps between the block particles, making gaps or the like less likely to be formed. As a result, the load life characteristic of the spark plug is improved. It is noted here that the softening temperature of glass refers to a temperature at which its coefficient of viscosity is 4.5xc3x97107 poise. If the softening temperature is less than 300xc2x0 C., heat resistance of the resistor is impaired. Therefore, it is preferable to use a glass material having a softening temperature of 300-800xc2x0 C., more desirably, 600-800xc2x0 C. In addition, different glass materials may be used between the coarse-particle glass (or block glass particles) and the fine-particle glass (or binding glass phase).
As to the softening temperature of glass, the softening point of the glass can be estimated through steps of analyzing the contents of oxidized element components such as B, Si, Ca, Ba and Li in the glass particles of the resistor, respectively, and calculating an oxide-equivalent composition, and obtaining a glass sample by blending, dissolving oxide raw materials for the individual element components to be oxidized so that the resulting composition becomes generally equal to the calculated composition, and thereafter quenching the raw material, where the softening point of the resulting glass sample is taken as the softening point of the relevant glass.
Also, desirably, the material of the glass particles is one whose difference between the softening temperature of the fine-particle glass and the softening temperature of the coarse-particle glass is not more than 100xc2x0 C. That is, desirably, if the softening temperatures of the fine-particle glass and the coarse-particle glass are TF and TC, respectively, then ¦TFxe2x88x92TC¦xe2x89xa6100xc2x0 C. In this case, it is allowable that either TF greater than TC or TF less than TC. The reason of this is given below.
In the first place, in comparison between fine-particle glass and coarse-particle glass, the former is more likely to be deformed in the hot press process than the latter even with the same coefficient of viscosity. Then, in the case where TF greater than TC, if ¦TFxe2x88x92TC¦xe2x89xa6100xc2x0 C., then even with the softening temperature of the fine-particle glass a little higher than that of the coarse-particle glass, the fine-particle glass is enough deformed by the pressure during the hot press so as to fill the gaps between the coarse-particle glass particles, allowing the load life characteristic of the spark plug to be held good. However, if ¦TFxe2x88x92TC¦xe2x89xa6100xc2x0 C., then the fine-particle glass may be deformed only insufficiently so that gaps are formed between the coarse-particle glass particles, which may cause a deterioration of the load life characteristic. On the other hand, in the case where TF less than TC, in which case the fine-particle glass is more likely to be deformed so that gaps are more unlikely to be deformed, if ¦TFxe2x88x92TC¦ greater than 100xc2x0 C., then the coefficient of viscosity of the glass becomes too low and besides voids due to foaming of the fine-particle glass are likely to occur to the conduction path forming part, which may cause a deterioration of the load life characteristic. Therefore, it is preferable that ¦TFxe2x88x92TC¦ is not more than 100xc2x0 C., and more desirably not more than 50xc2x0 C.
Next, the resistor composition may contain at least either one of a metallic phase principally comprising Ti as the conductive material (hereinafter, referred to as Ti-based metallic phase) and titanium suboxide particles represented by a composition formula of TinO2nxe2x88x921 as the semiconductive ceramic particles. It is noted that titanium suboxide herein referred to is a titanium oxide having an oxygen content lower than titanium dioxide, and can also be represented by a composition formula of TiOx (x less than 2).
The anatase type TiO2 conventionally blended in the resistor composition is semiconductive and has a property that the electric resistance decreases with increasing temperature (i.e., having a negative temperature coefficient). In this case, because of a relatively large rate of change of the electric resistance due to temperature increase, the electric resistance at high temperatures decreases to a large extent so that excessively increasing the blending amount would cause the radio frequency noise prevention performance to be impair, as a disadvantage. In contrast to this, the aforementioned titanium suboxide, similarly semiconductive as it is, is smaller in the rate of change of electric resistance due to temperature increase than titanium dioxide, so that decrease in the electric resistance of the resistor at high temperatures is suppressed and, as a result, a successful radio frequency noise prevention performance can be ensured even at high temperatures. Further, the Ti-based metal increases in electric resistance with increasing temperature, conversely (i.e., having a positive temperature coefficient), and therefore is capable of producing the same effects for the suppression of resistance decrease at high temperatures as in the aforementioned titanium suboxide. Further, the Ti-based metallic phase and the titanium suboxide particles in the resistor composition act also as load life stabilizers, thus allowing an effect of improving the load life characteristic of the resistor to be achieved as well. In addition, the Ti-based metallic phase and the titanium suboxide may be contained in the resistor composition either singly, whichever it is, or in combination of both.
In this case, when the total content of the Ti-based metallic phase and/or the titanium suboxide particles in the resistor composition is adjusted to a range of 0.5-10 weight %, the aforementioned effect can be made even more remarkable. If the total content is less than 0.5 weight %, then the effect of suppressing resistance increase at high temperatures may be insufficient in some cases. Also, if the total content exceeds 10 weight %, excessive increase in the specific electrical resistivity of the resistor composition may be caused.
Preferably, the Ti-based metallic phase and/or the titanium suboxide particles is adjusted so as to have a mean particle size of 5 xcexcm-100 xcexcm. If the mean particle size is less than 5 xcexcm, the Ti-based metallic phase and/or the titanium suboxide particles are more likely to progress in oxidation reaction during the production of the resistor, so that the effect of suppressing resistance increase at high temperatures becomes insufficient in some cases. On the other hand, if the mean particle size exceeds 100 xcexcm, excessive increase in the specific electrical resistivity of the resistor composition may be caused in some cases. In addition, the mean particle size is preferably adjusted within a range of, desirably, 10-30 xcexcm.
For the present invention, the titanium suboxide particles may principally comprise at least any one of TiO (cubic crystal system), Ti2O3 (hexagonal crystal system) and Ti3O5 (monoclinic crystal system). Out of these, Ti3O5 is particularly preferable for the present invention by virtue of its stability to humidity, atmosphere and the like. In addition, although composition formulas of various titanium suboxides are represented all by stoichiometric ratio, but they may become nonstoichiometric compositions due to oxygen deficiency in some cases.
The ceramic particles other than the titanium suboxide may be those composed principally of, for example, one kind or more selected from a group of ZrO2, ZrSiO4, Al2O3, MgO, Alxe2x80x94Mg spinel and mullite.
The resistor composition may comprises 2-60 weight % of glass, 2-65 weight % of the ceramic particles (including titanium suboxide), and 0.1-7 weight % of carbon component. Such a resistor composition can be obtained, for example, by preparing a raw material powder through the steps of mixing 2-60 weight % of glass particles, 2-65 weight % of ceramic particles (including titanium suboxide), 0.1-5 weight % of a non-metallic conductive material (e.g., carbon black) and 0.1-5 weight % of organic: binder (e.g., PVA) plus, as required, an appropriate amount of metal powder (which results in a metallic phase), and molding and heating this raw material powder.
Specifically, the blending ratio of the raw material powder of the resistor composition is preferably as follows:
fine-particle glass: 0.5-20 weight %;
coarse-particle glass: 50-90 weight %;
Ti metal particles and/or titanium suboxide particles: 0.5-10 weight %;
auxiliary ceramic particles: 0.1-6 weight % and
non-metallic conductive material particles: 0.5-7.0 weight %.
Also, from the viewpoint of structure, the resistor composition preferably comprises: 50-90 volume of the aforementioned block glass particles and 10-50 volume % of the conduction path forming part. In addition, the conductive material particles contained in the conduction path forming part may contain a metallic phase principally comprising one kind or more selected from among Al, Mg, Ti, Zr and Zn, and a non-metallic conductive material.
Also, the volume ratio of the Ti-based metallic phase or the titanium suboxide particles occupying in the conduction path forming part is preferably adjusted to within a range of 5-50 volume %, desirably, 20-40 volume %. If the volume ratio is less than 5 volume %, then the resistor becomes insufficient in load life characteristic. If it exceeds 50 weight 0.5 its noise prevention performance is more likely to deteriorate due to high temperature.
In this case also, the non-metallic conductive material particles may be amorphous carbon (carbon black), and besides graphite, SiC, TiC, WC, ZrC or the like. Preferably, the content of carbon in the resistor composition is adjusted within a range of 0.5-7.0 weight % as stated before. If the carbon content is less than 0.5 weight %, then the load life characteristic of the spark plug is impaired in some cases. Also, if the carbon content exceeds 7.0 weight %, then the radio frequency noise prevention effect becomes insufficient in some cases. The carbon content is preferably adjusted within a range of, more desirably, 2.0-5.0 weight %.
A second constitution of the resistor-incorporated spark plug is characterized in that the resistor composition contains at least one of TiC particles and TiN particles as a non-metallic conductive material.
The resistor of a spark plug is exposed to severe conditions such as high voltages and high temperatures and, as a result, progressively oxidizes as the time in use elapses. It is noted here that although the aforementioned carbon black has often been used as the non-metallic conductive material hereto, carbon black would change into CO or CO2 and dissipate when oxidized, so, that the resistance value may abruptly increase with progressing oxidation. However, using at least one of the TiC particles or the TiN particles in place of carbon black or together with carbon black offers the following advantage. That is, TiC or TiN will not dissipate even if oxidized, and yet forms semiconductive TiO2 (or titanium suboxide), so that any abrupt increase in the resistance value can be suppressed. Further, TiC or TiN generally has a large particle size on the order of several xcexcm (10 to 100 times that of carbon black particles), thus requiring a long time until it is completely oxidized. Therefore, a spark plug which is less in change with time of the resistor and superior in durability can be obtained.
In this case, the total content of the TiC particles and/or the TiN particles in the resistor composition is preferably set within a range of 1-10 weight %. If the total content is less than 1 weight %, then the absolute content of conductive material lacks so that an increase in the initial resistance value may be incurred. Also, because of a thinned conduction path, the load per unit area becomes higher so that the durability may deteriorate. On the other hand, if the total content exceeds 10 weight %, the initial resistance value becomes too low so that the expected radio frequency noise prevention performance could no longer be obtained.
When the TiC particles and/or the TiN particles in the resistor composition have a mean particle size of not more than 5 xcexcm in a particle image obtained from observation of its cross-sectional structure, enough specific surface area of the TiC particles and/or TiN particles per unit area of the resistor can be ensured so that variation with time in the resistance value is lessened and the durability of the resistor can be improved. Furthermore, it becomes easy to adjust the resistance value of the resistor to the expected target value.
Further, oxygen content of the TiC particles and/or the TiN particles is preferably not more than 3 weight %. In other words, TiC particle and/or TiN particles, which serve as the starting material of the resistor composition, are preferably those having an oxygen content of not more than 3.0 weight %. If the oxygen content exceeds 3.0 weight %, then the oxygen concentration at surface layer portions of the particles increases so that the contact resistance between particles themselves becomes high, which may cause a deterioration of the durability of the resistor.
The resistor composition may contain 20-80 weight % of glass, and 2-60 weight % of the ceramic particles. Such a resistor composition can be obtained, for example, by preparing a raw material powder through the steps of mixing 1-10 weight % of TiC particles and/or TiN particles, 20-80 weight % of glass powder, 2-60 weight % of ceramic powder, 0.5-5 weight % of organic binder (e.g., PVA) plus, as required, an appropriate amount of metal powder (which results in a metallic phase) or a non-metallic conductive material (e.g., carbon black) other than the TiC particles and/or TiN particles, and heating and molding this raw material powder.
In this case, specifically, the blending ratio of the raw material powder of the resistor composition is preferably as follows:
fine-particle glass: 0.5-20 weight %;
coarse-particle glass: 50-90 weight %
ceramic particles: 2-60 weight % and
non-metallic conductive material particles (including TiC particles and/or TiN particles): 1-10.0 weight %.
Also, from the viewpoint of structure, the resistor composition preferably comprises: 50-90 volume % of the aforementioned block glass particles and 10-50 volume % of the conduction path forming part. In addition, the conductive material contained in the conduction path forming part may contain a metallic phase principally comprising one kind or more selected from among Al, Mg, Ti, Zr and Zn, and the non-metallic conductive material.
Also, the volume ratio of the TiC particles and/or the TiN particles occupying in the conduction path forming part is preferably adjusted within a range of 5-50 volume %, desirably, 20-40 volume %. If the volume ratio is less than 5 volume %, then the resistor becomes insufficient in load life characteristic. If it exceeds 50 volume %, its noise prevention performance is more likely to deteriorate due to high temperature.
In addition, when a carbon base conductive material such as carbon black or graphite is blended in addition to the TiC particles and/or the TiN particles, it is preferable that the content of the carbon component in the resistor composition except those contained in the TiC particles is not more than 7.0 weight %. If this content exceeds 7.0 weight %, then the radio frequency noise prevention effect may become insufficient in some cases.
Next, a third constitution of the spark plug as well as a process for manufacturing the same according to the present invention are characterized in that a resistor composition constituting a resistor is fabricated by using a raw material powder which principally comprises glass particles, ceramic particles other than glass, and carbon black particles having a mean particle size of 20 nm-80 nm.
The carbon black intervenes is intervenient among the other raw material powder (glass, ceramic) particles in the resistor, and primary particles of the carbon black concatenate in a one-dimensional fashion to form concatenated structures. The resulting structures are further connected to one another, thus forming a conductive network of the resistor.
When the raw material powder of the resistor is prepared by wet mixing with the use of an aqueous medium, carbon black is poor in dispersibility because of the factors such as low wettability with water of large specific gravity. In particular, when the carbon black is small in particle size or long in structure, it becomes difficult to obtain a uniform distribution of carbon black. As a result, the carbon black is maldistributed in the resistor composition, causing a problem that when glass is sealed with this resistor composition, the resulting resistor is varied in resistance value and besides the conduction path is localized with the result of concentrated current densities, hence an unstable load life characteristic of the spark plug. On the other hand, when the carbon black is too large in particle size or short in structure, the conductivity decreases so that the blending amount of carbon black needs to be increased. However, because carbon black is far smaller in particle size than the other raw material powders such as glass and ceramic powders, excessive increase of the blending amount would cause the bulk density of the raw material powder to increase so that bridging of the powder particles or the like becomes more likely to occur, resulting in a loss of the compressibility. As a result, the resistor obtained would result in a lower density as well as an increased amount of defects such as voids, causing a problem of an unstable load life characteristic of the spark plug.
The present inventors have energetically discussed in view of these standpoints, finding out that by using a carbon black having a mean particle size of 20 nm-80 nm, the resistor obtained can be made less in variation of resistance value and the spark plug using this resistor can be stabilized in load life characteristic.
It is due to the following reason that the mean particle size of the carbon black is restricted to the range of 20-80 nm. First, a mean particle size of 20 nm or more allows the carbon black to be uniformly distributed into the resistor composition, so that variation in the resistance value of the resistor can be suppressed, and besides that the current path is dispersed, making the concentration of current densities unlikely to occur. on the other hand, a mean particle size of 80 nm or less makes it possible to obtain a successful conductivity even with a reduced blending amount of the carbon black. As a result, the amount of use of carbon black, which is finer as compared with the other raw material powders, can be reduced so that the bulk density of the raw material powder for the resistor composition can be enhanced, with the results that the resistor finally obtained is enhanced in density and moreover that a resistor less in defects and stable load life characteristic can be obtained. In addition, the mean particle size of carbon black is preferably within a range of, desirably, 30-50 nm.
In this case, preferably, the carbon black powder is a powder whose amount of DBP (dibutylphthalate) absorbed by 100 g of carbon black as defined in A process of Japanese Industrial Standard K6221, 6.1.2 is 60-120 ml. This amount of DBP absorption, which increases with increasing structure length in the carbon black powder, can be used as an index that reflects the structure length (hereinafter, the amount of DBP absorption measured by this process will be referred to as xe2x80x9cstructure lengthxe2x80x9d.)
Then, when the structure length of carbon black is not more than 120 ml/100 g, the structure can be uniformly distributed into the resistor and the current path is dispersed, making the concentration of current densities unlikely to occur. On the other hand, when the structure length is not less than 60 ml/100 g, it becomes possible to obtain a successful conductivity with a less blending amount of carbon black, so that the amount of use of carbon black is reduced and that the bulk density of the raw material powder for the resistor composition can be enhanced. As a result, the resistor finally obtained is improved in density so that a resistor less in defects and stable in load life characteristic can be obtained. In addition, preferably, the structure length is within a range of, desirably, 80-100 ml/100 g.
In this case, preferably, the raw material powder of the resistor composition comprises 20-90 weight % of glass powder, 20-50 weight % of ceramic powder, 5-30 weight % of carbon black powder, and 0.05-5 weight % of an organic binder. If the blending amount of glass powder is less than 20 weight %, then it may be impossible to ensure a successful sealability. On the other hand, if it exceeds 90 weight %, then the load life characteristic may become insufficient. The blending amount of glass powder is preferably within a range of 70-80 weight %. Meanwhile, if the amount of ceramic powder is less than 20 weight % or if the amount of carbon black powder is less than 5 weight %, the conduction path may become excessively thin so that a deterioration of the load life may be incurred. Also, if the amount of ceramic powder exceeds 50 weight % or if the amount of carbon black exceeds 30 weight %, then a deterioration of radio frequency noise prevention performance results. In addition, preferably, the amount of the ceramic powder is within a range of 20-30 weight % and the amount of carbon black is within a range of 5-10 weight %.
For the resistor compositions of the present invention, it is preferable that the specific electrical resistivity at 20xc2x0 C. is adjusted within a range of 50-2000 xcexa9xc2x7cm. If the value of specific electrical resistivity is less than 50 xcexa9xc2x7cm, then the noise prevention performance may become insufficient. Also, if the value of specific electrical resistivity exceeds 2000 xcexa9xc2x7cm, then the load life characteristic may become insufficient. The value of the specific electrical resistivity is preferably adjusted within a range of, more desirably, 100-1200 xcexa9xc2x7cm.
A fourth constitution of the resistor-incorporated spark plug according to the present invention is characterized in that the resistor composition contains, as the ceramic particles, 0.5-20 weight % of TiO2 particles whose mean particle size of a particle image obtained from observation of its cross-sectional structure falls within a range of 0.5-20 xcexcm, the TiO2 particles in the resistor composition at least partly having a rutile type crystalline structure.
Further, a fifth constitution of the resistor-incorporated spark plug according to the present invention is characterized in that the resistor composition contains, as the ceramic particles, 0.5-20 weight % of at least either one of a semiconductive titanate base complex oxide and a semiconductive zirconate base complex oxide (specific complex oxide).
A sixth constitution of the invention is characterized in that the resistor composition contains at least either one of a metallic phase principally comprising Ti as the conductive material (hereinafter, referred to as Ti-based metallic phase) and titanium suboxide particles represented by a composition formula of TinO2nxe2x88x921 as the ceramic particles.